1. Field of the Invention
The present invention relates to an acceleration sensor for use in measurement of an acceleration, detection of vibration, etc.
2. Related art of the Invention
Recent years have seen further size reduction of electronic devices and a more popular use of portable electronic devices such as a notebook computer. To ensure and improve the reliability of such an electronic device against an impact, a small high-performance acceleration (shock) sensor which enables surface mounting is increasingly desired. For example, when an impact is applied during writing in a high-density hard disk, a head is displaced. As a result, a data write error may be created or the head may be damaged. To deal with this, it is necessary to detect the impact which is applied to the hard disk and stop the write operation or retract the head to a safe position.
Meanwhile, there is also an increasing demand for an acceleration sensor for shock detection which is to be used in an airbag device for protecting a passenger from an impact upon a car crash accident, and for detection of an acceleration for suspension control. These acceleration sensors are desired to be further reduced in size and weight.
By the way, piezoelectric ceramics can be used as an acceleration sensor, since when force which is proportional to an acceleration is applied to a piezoelectric ceramic, the piezoelectric ceramic is internally distorted and an electric charge is created at the both surfaces of the piezoelectric ceramic. FIG. 38 is a drawing which shows a structure of a piezoelectric ceramic element 500 of a conventional piezoelectric type acceleration sensor. The structure is a metallic plate joined to a piezoelectric ceramic 50 which is formed in a plate-like or disk-like shape. Electrodes are disposed on the two surfaces, one upper and the other lower, of the piezoelectric ceramic 50. The electrode disposed on one of the two surfaces is divided into two, and used as a drive electrode 52 and a detection electrode 51 for self diagnosis. As the piezoelectric ceramic element 50 deflects and vibrates in the vertical direction, an electric charge is created at an output electrode.
FIG. 39 shows a typical signal processing circuit of a piezoelectric type acceleration sensor. For an acceleration sensor for measuring an acceleration, a source follower circuit which uses an field effect transistor (FET). The source follower circuit has a large impedance conversion efficiency, and the gain of the circuit is about 0 dB.
In the circuit shown in FIG. 39, an output frequency range on a lower frequency side is determined by an electrostatic capacitance C11 of the acceleration sensor and a cut-off frequency which is defined by a time constant (1/.omega.s) of a high pass filter which is formed by a resistor Rh which is connected in parallel to the acceleration sensor. The cut-off frequency fhc of the high pass filter is: EQU fhc=1/.omega.s=1/(2.pi..multidot.C11.multidot.Rh)
Although the capacitance of a piezoelectric type acceleration sensor is generally dependent on the shape of the acceleration sensor, a piezoelectric sensor which is fabricated using a piezoelectric ceramic has a few hundreds pF. On the other hand, the resistor Rh has about 1 M to 10 M.OMEGA. when a general purpose chip resistor which uses the resistor Rh as a gate resistor is used. Hence, in the case of a piezoelectric sensor which uses a piezoelectric ceramic, the cut-off frequency is about a few hundreds Hz.
As described above, the lower limit of a measurable frequency of the piezoelectric type acceleration sensor is determined by a capacitance of the acceleration sensor and a resistance value which is connected thereto, which lowers an output on the lower frequency side.
Next, principles of self diagnosis will be described below. A self diagnosis pulse is applied to the drive electrode for self diagnosis of the piezoelectric element from a transmitter. In response to the self diagnosis pulse, the piezoelectric element vibrates, and the vibration vibrates the acceleration sensor. At this stage, an electric charge which is in accordance with the scale of the vibration is developed at the detection electrode, and converted by the signal processing circuit into a voltage. When the voltage which is obtained as a result of detection of the vibration which is created by the self diagnosis pulse is different from a value which is set in advance, the signal processing circuit diagnoses that the acceleration sensor is in an abnormal condition and executes procedures for dealing with abnormality. In this manner, it is possible to add a function of self-diagnosing a malfunction of the acceleration sensor.
By the way, when an acceleration sensor using a piezoelectric ceramic is to be used in a safety device such as an airbag or to improve the comfortability while driving a rocky road, demanded characteristics are a capability of detecting an acceleration to as low a frequency range as possible, an excellent detection sensitivity in a detectable range, a small pyroelectric effect, and a self diagnosis capability.
To detect an acceleration to as low a frequency range as possible, it is necessary that the cut-off frequency described above is small. Hence, it is necessary to increase the capacitance of the acceleration sensor and the resistance value which is connected to the acceleration sensor.
Against this backdrop, one approach is to increase the resistor Rh which is connected in parallel to the acceleration sensor, thereby reducing the cut-off frequency fhc and accordingly obtaining a constant output to a low frequency. However, a resistor exceeding 10 M.OMEGA. is very expensive, and therefore, this approach is not realistic.
Meanwhile, unless a special consideration is taken on a leak current or the like between wires disposed on a substrate to mount, a high resistance can not be substantially obtained. A member, such as a guard ring, for preventing a leak current must be disposed to a connection terminal of the resistor member to the substrate. There is a problem that a change in the environment such as humidity changes a leak current between the wires of the substrate so that an apparent resistance value becomes small.
In addition, although it is easy to form the signal processing circuit which is shown in FIG. 39 as an integrated circuit, like the resistor Rh, a high-resistance resistor of 10 M.OMEGA. or more is very difficult to incorporate in an integrated circuit with the current semiconductor technology. While this forces to prepare a high-resistance resistor separately from the acceleration sensor and the signal processing circuit and mount the high-resistance resistor, this increases the number of mounting parts and expands a mounting area size, thereby preventing an effort to reduce the size of a shock detection apparatus or the like.
An alternative approach is to increase the electrostatic capacitance C11 of the acceleration sensor. When the material is same, an electrostatic capacitance or an acceleration sensor becomes larger as the thickness of a piezoelectric element is thinner and the area size is larger. Nevertheless, if the piezoelectric element is formed thin, the mechanical strength of the piezoelectric element is decreased and the piezoelectric element easily breaks, and it becomes hard to treat the piezoelectric element during manufacturing steps. Meanwhile, if the piezoelectric element is formed to have a large area size, it becomes difficult to realize a compact size. Further, since the shape of the piezoelectric element determines a resonance frequency which is closely related with a measurement frequency band, it is not easy to change the shape of the piezoelectric element, which is a problem. Hence, the capacitance should be increased by a different approach.
On the other hand, even if detection within a predetermined frequency range is possible, a sensitivity of the detection must be high. By the way, when the sane piezoelectric ceramic is used, a quantity of electric charges which are generated by vibration is constant. Since Q (electric charges)=C (capacitance) V (sensitivity), the sensitivity is higher as the capacitance is smaller.
That is, to allow the acceleration sensor to detect in as low a frequency range as possible and have a high sensitivity, it is necessary that the capacitance is large and the size is large as well.
Further, with respect to the sensitivity, if electric charges generated during vibration are small, few, it is necessary to use amplifiers in two stages to amplify a signal, which makes the circuit complex. At the same time, since a noise is similarly amplified, the SN ratio deteriorates. Therefore, to prevent a malfunction, the circuit becomes even more complex.
Still further, in the case that the transmitter supplies an oscillation pulse to the drive electrode for the purpose of self diagnosis, the oscillation pulse is driven at a low frequency and vibration is detected by the detection electrode, since the capacitance of the piezoelectric element at the vibration detection electrode portion is small as described earlier, the output voltage from the vibration detection electrode is small. Since the vibration detection electrode is not disposed all over the piezoelectric element, the output voltage is smaller even than the capacitance of the piezoelectric element as a whole. In addition, a quantity of electric charges which are generated at the vibration detection electrode is small depending on the surface area size.
As the output voltage at a low frequency for detection of the vibration during self diagnosis becomes small, the accuracy of self diagnosis at a low frequency deteriorates. This leads to a problem that it is not possible to accurately perform self diagnosis in all frequency band ranges.
Further, vibration fails to be detected if the surface area of the self diagnosis electrode is too small, whereas vibration intensifies and the piezoelectric element is destroyed if the surface area of the self diagnosis electrode is too large. However, when the surface area of the self diagnosis electrode is reduced and the gap from the detection electrode accordingly becomes large, a portion without an electrode expands in the element. If there are a portion with an electrode and a portion without an electrode in the surface of the piezoelectric element layer, due to a difference in the coefficient of contraction, the piezoelectric element bends or rolls, which makes it impossible to obtain a stable element without bending.